This invention relates to a device for detecting the presence of fluids in conduits or containers. In one aspect the invention relates to an optical sensor system which is responsive to the index of refraction of the fluid. In another aspect, the invention relates to an optical sensor system which can distinguish between fluids having different indices of refraction including vapors and liquids.
The ability to remotely monitor fluids is important in many industrial applications. Moreover, there are a number of applications which require the ability to differentiate between fluids. Liquid level detectors, and the like, represent common examples of systems which operate on the principle of differentiating between fluids. In another example, product pipelines are often used to transport different fluids in slugs or batches, and it is important to identify the interface between the batches so that valving can be controlled to minimize cross-contamination of the fluids. Other applications include leak detection in the interstitial space of double-walled containers used to store hydrocarbons or other petrochemicals. In interstitial leak monitoring, water is often intermittently present due to condensation or the invasion of rain and snow, and the ability to distinguish water from a hydrocarbon would eliminate false leakage alarms, and provide an indication of the actual condition of the inner lining of the container.
A number of devices which employ optical-type sensors for the purpose of remotely detecting the presence of fluids in pipelines, tanks, vessels, and the like have been developed. Typically these devices comprise the components of a light source, an optical-type probe placed at the location where the fluid is to be detected, a light detector element, and fiber optics which optically connect the source, the probe, and the detector in series. In normal operation, the source will send light to the probe via the transmission line and the probe will in turn transmit light to the detector at a predetermined intensity level thus indicating normal operation. For detecting the presence of a fluid, the probe will have an optical property which undergoes a change as its environment changes from one fluid to another (e.g. from air to water), so that the light returned from the probe to the detector will be attenuated or otherwise disrupted to indicate the presence of the fluid to be detected.
U.S. Pat. No. 5,187,366 describes an optical sensor which operates on the principles described above and is designed to detect leakage into the interstitial spaces of double-walled containers. The probe comprises a first and second optic fibers aligned in end-to-end relation and separated by a small gap. In normal operation light emitted by the source travels through the first optic fiber, across the gap, and through the second fiber to the detector. The fiber optic ends of the probe are held in alignment using a coating of a material which is soluble in the fluid to be detected. Upon exposure to the fluid the coating dissolves and the ends of the first and second fibers are induced to move out of optical alignment, thereby disrupting the flow of light from the emitter to the detector indicating the presence of the fluid. A mechanical spring is used to bias the probe fiber ends apart to ensure the misalignment of the fibers as the coating dissolves.
U.S. Pat. No. 5,200,615 describes an optical sensor which uses a probe comprising a first and second optic fibers having ends in parallel relation which are embedded within a soluble phosphorescent coating. Light sent to the probe from the source via the first fiber is absorbed by the phosphorous material and re-emitted. The re-emitted light illuminates the end of the second fiber and travels therethrough to the detector to indicate normal operation. Upon exposure to the fluid to be detected the soluble coating dissolves and the phosphorous material is dispersed thereby disrupting the flow of emitted light to the detector.
U.S. Pat. No. 5,200,615 further teaches the use of a probe comprising a single optic fiber having a core and cladding. The probe is formed by stripping a small length of the cladding off to expose the core, and applying a phosphorescent soluble coating around the core. As the coating dissolves and the phosphorous material is disbursed the light intensity at the detector is attenuated thereby indicating the presence of the fluid. A disadvantage of the sensors of U.S. Pat. 5,187,366 and 5,200,615 is that the soluble coating must be replaced after exposure to the fluid to be detected.
An optical device for measuring the level of liquid in a container is described U.S. Pat. No. 4,641,025. The device comprises a light pulse source such as a laser diode, an on/off type optical detector, and a plurality of probes connected between the source and detector by fiber optics. Each probe attenuates the light traveling from the source to the detector by an amount proportional to the index of refraction of the fluid surrounding the probe. A plurality of such probes are positioned along a line which intersects the boundary between the liquid and vapor in the container, so that in operation a number of probes will be positioned in the vapor above the liquid (typically air) and the remaining probes will be immersed in the liquid. The light pulse source is provided with a timing circuit to send light pulses of equal intensity to the probes sequentially, producing an attenuated sequence of pulses transmitted to the detector. Because the vapor has a lower index of refraction than the liquid, the intensity of the light transmitted to the detector via the probes in the vapor is higher than the light transmitted via the probes in the liquid. The detector is provided with a threshold circuit such that the detector produces an outgoing electrical analog pulse corresponding only to an incoming light pulse having an intensity above a predetermined threshold. Light pulses received below the threshold intensity produce no analog pulse. The threshold is set between the intensity of the light pulses transmitted by the probes in the vapor and that transmitted by the probes in the liquid, and thus only probes located in the vapor result in an electrical analog pulse at the detector. The analog pulses can be counted to determine the number of probes in the vapor and thereby determine the location of the liquid level. The device of U.S. Pat. No. 4,641,025 would not appear to be directly applicable to monitoring fluids in a flowing environment such as a pipeline.